Turret-type machine tool metal working punch presses have been in use for many years and have enjoyed significant commercial success because they dramatically reduce the punching time in cases where multiple punching operations are performed on a single workpiece. And with the advent of numerical controls for these turret punch presses, the total workpiece punching time has been decreased significantly further.
In many turret-type punch presses, the tool pair, i.e., the punch and the die, frequently include a punch guide mounted for limited reciprocal motion within the turret and biased into the turret by a coil compression spring. This guide serves to: (a) clamp the workpiece down against the die which is mounted in the turret under the workpiece; (b) guide the punch itself which is reciprocably mounted in the guide, and (c) strip any excess material from the punch as the punch withdraws from the workpiece into the forward end of the guide.
Typically, the punch is an elongated cylindrical body having a reduced inner end that treadedly receives a spring seat for a coil compression stripper spring that biases the punch away from the work into the guide. The forward end of the punch is also reduced and defines the metal cutting shearing surfaces, and the shape of this end of the punch, while sometimes merely circular, frequently has a non-circular configuration of practically any shape desired.
If the punch in these prior designs needs replacing because of tip wear or breakage, it is necessary to withdraw the entire tool assembly from the turret which requires some disassembly, and then remove the old punch from the guide by unthreading the stripper seat from the end of the punch and then removing the punch and replacing it and reassembling it into the guide, and replacing the resulting assembly back in the turret. The entire broken or worn punch is then discarded. The punch itself is a completely machined part including a central guide portion with frusto-conical transitions on both ends, a reduced inner stem portion, with threads at the proximal end. For this reason the punches are quite costly and because replacement is frequent, it adds significantly to the resulting part cost. Furthermore, the punches are usually hardened by heat treating, and the rather large size of the punches results in a substantial heat treating cost even though only the tip of the punch requires heat treating.
There have, in the past, been attempts to construct punches in two pieces, that is, a holder and a separate punching element held by a holder, but these attempts have either resulted in a more costly punching assembly or one in which the punching element is not easily replaceable. These prior attempts have also not been applied to punches in turret presses and, furthermore, the guide elements for these prior punches do not guide both the holder and the punching element itself.
In the Smith, U.S. Pat. No. 2,580,366, a punch holder is shown in which a punch is insertable from the rear of the holder and held in position by a back-up rod. This assembly is extremely costly and the punching element itself is difficult to replace because of its rear mount.
The Good, et al., U.S. Pat. No. 2,703,143 shows a punch holder with a forwardly insertable punch held in position by a pair of angularly related set screws in the holder that engage chamfers on both sides of the punching tool. Both set screws must be removed to replace the tool and the tool itself is completely unguided which promotes tool breakage.
The Whistler, Sr., et al., U.S. Pat. No. 3,137,193, shows a two piece punch, but the punch itself is unguided and is essentially as large as the present punch and holder combined and therefore, results in no cost saving.
The Vecchi, U.S. Pat. No. 3,274,878, shows a one piece punch and holder.
The Saladin, U.S. Pat. No. 3,741,056, shows a two piece punch but the punch essentially is the size of the punch and holder combined.
The Stursberg, U.S. Pat. No. 3,763,732, shows essentially a one piece punch similar to the Saladin punch.
The Teeslink, U.S. Pat. No. 4,240,314, the Johnson, U.S. Pat. No. 3,063,319, and the Knehans, U.S. Pat. No. 3,527,130, all show two piece punch assemblies but the punch itself is the size of the prior one piece punches and hence, results in no cost saving whatsoever.
It is a primary object of the present invention to provide a punch assembly for a turret punch press that ameliorates the problems noted above in prior art punch assemblies.